1. Field of the Invention
The present invention relates to a chemically-amplified positive resist composition used for processing a semiconductor and a photo mask blank, and a resist patterning process using the same.
2. Description of the Related Art
In recent years, as an integrated circuit progresses toward a high integration, a further finer patterning is required. When a resist pattern of 0.2 μm or less is processed, as a usual manner, a chemically amplified resist composition in which an acid acts as a catalyst has been used. As the exposure light source, a high energy beam such as an ultraviolet beam, a far-ultraviolet beam and an electron beam has been used, and especially, electron beam lithography, which is used as ultrafine processing technique, has become indispensable as a method of processing a photo mask blank in producing a photomask used for semiconductor manufacturing.
Polymers having a large amount of aromatic skeleton with acidic side chain, such as polyhydroxystyrene, have been favorably used as a resist composition for KrF excimer laser, but not for ArF excimer laser because they exhibit large absorption to light whose wavelength is approximately 200 nm. Due to high etching resistance, however, this type of polymer essentially serves as a useful resist composition for electron beam to form a finer pattern than a processing limit using an ArF excimer laser and as a resist composition for extreme ultraviolet (EUV).
Normally, a base polymer of a positive resist composition for electron beam or a resist composition for EUV is mainly composed of a material that solubilizes in an alkaline developer. Specifically, the solubilization is performed by subjecting an acid-soluble protective group masking an acid functional group of a phenol side chain in a base polymer to deprotection with use of an acid generated from a photo acid generator by high energy beam exposure as a catalyst. In addition, the acid-soluble protective group mainly includes a tertiary alkyl group, a t-butoxycarbonyl group and an acetal group. Herein, use of a protective group having a relatively low activation energy required for deprotection such as an acetal group can advantageously provide a resist film with high sensitivity. However, if acid diffusion generated is insufficiently controlled, a deprotection reaction is caused even in a non-exposed area in a resist film, leading to such technical problems as deterioration of a line edge roughness (LER) and decline in in-plane uniformity (CDU) of a pattern line width.
To control the resist sensitivity and pattern profile, various improvements have been achieved according to the way materials used in a resist composition are selected, combined and processed. One of its improvements is to control acid diffusion. Acid diffusion has been extensively discussed, because it can significantly affect sensitivity and resolution of a chemically-amplified resist.
Patent Documents 1 and 2 describe an example of controlling acid diffusion to reduce roughness by making bulky a benzenesulfonic acid generated from a photo acid generator by exposure. However, since the degree of the acid diffusion control remains insufficient, development of an acid generator with reduced diffusion has been desired.
In addition, Patent Document 3 describes an example of controlling acid diffusion by bonding a sulfonic acid generated by exposure to a resin used in a resist composition. The method for controlling acid diffusion by bonding a repeating unit that generates an acid by exposure to a base polymer is effective in obtaining a pattern with low LER. However, according to the structure and introduction ratio of the repeating unit, a base polymer obtained by bonding the repeating unit that generates an acid by exposure has a problematic solubility to an organic solvent.
In addition, use of a sulfonium salt that generates an acid having a high acid strength such as fluorinated alkanesulfonic acid and a resin comprising a repeating unit having an acetal group described in Patent Document 4 can form a pattern with a high LER. Specifically, since the acid strength of fluorinated alkanesulfonic acid is too high for deprotection of an acetal group having a relatively low activation energy therefor, even control of acid diffusion can make a progressive deprotection reaction by a slight amount of an acid that has diffused into a non-exposed area. This is also found in a sulfonium salt that generates a benzenesulfonic acid described in the Patent Documents 1 and 2. Therefore, development of an acid generator that generates more desirably strong acid for deprotection of an acetal group is being desired.
In order to control acid diffusion, improvement in acid diffusion control agent (called also as “quencher”) is required, in addition to the method for making bulky the acid generated.
Acid diffusion control agent is substantially essential component to control acid diffusion and improve resist performance. Development of an acid diffusion control agent has conventionally been discussed in various manners, and amine and acidulous onium salt are commonly used. As for an example of the acidulous onium salt, Patent Document 5 describes that the addition of triphenyl sulfonium acetate enables a favorable resist pattern without T-top shape, line width difference between a dense pattern and an isolated pattern, and standing wave to be formed. Patent Document 6 describes improvement in sensitivity, resolution and exposure margin by addition of a sulfonic acid organic salt or a carboxylic acid organic salt. Patent Document 7 describes that a resist compositions for KrF and electron beam that contain a photo acid generator generating a fluorine-containing carboxylic acid is excellent in resolution, and improves process allowability such as exposure margin and focal depth. In these technologies, a strong acid generated from other photo acid generators by exposure (sulfonic acid) exchanges with an acidulous onium salt to produce weak acid and strong acid onium salt. Accordingly, by substituting a strong acid of high acidity (sulfonic acid) with a weak acid (carboxylic acid), acid decomposition reaction of an acid-labile group is controlled to achieve a smaller interval for acid diffusion. Apparently, the resulting product functions as an acid diffusion control agent.
However, use of the-described resist composition containing a carboxylic acid onium salt or a fluorocarboxylic acid onium salt for patterning can cause a higher LER. Thus, development of an acid diffusion control agent capable of reducing LER has been desired.